Fuel cells may be used for environmentally friendly generation of electricity. A process which essentially represents the converse of electrolysis takes place in a fuel cell. A fuel which includes hydrogen is supplied to an anode in a fuel cell, and an auxiliary substance which includes oxygen is supplied to a cathode. The anode and cathode are in this case electrically isolated from one another via an electrolyte layer, in which case, although the electrolyte layer allows ions to be exchanged between the fuel and the oxygen, the electrolyte layer otherwise ensures that the fuel and the auxiliary substance are separated in a gastight manner. As a consequence of the exchange of ions, hydrogen which is included in the fuel can react with the oxygen to form water, with electrons being enriched on the fuel-side electrode or anode, and electrons being absorbed on the auxiliary substance side electrode or cathode.
During operation of the fuel cell, a usable potential difference or voltage is thus formed between the anode and cathode, with the only waste product from the electricity generation process being water. The electrolyte layer which, in the case of a high-temperature fuel cell, may be in the form of a ceramic solid electrolyte, or in the case of a low-temperature fuel cell may be in the form of a polymer membrane, thus has the function of separating the reactants from one another, of carrying the charge in the form of ions, and of preventing an electron short circuit.
Owing to the electrochemical potentials of the substances that are normally used in a fuel cell such as this, an electrode voltage of about 0.6 to 1.0 V can be formed in normal operating conditions, and can be maintained during operation. For technical applications in which a considerably higher total voltage may be required depending on the purpose or the planned load, a number of fuel cells are thus normally connected electrically in series in the manner of a fuel cell stack, such that the sum of the electrode voltages produced by each of the fuel cells corresponds to or is greater than the required total voltage. Depending on the required total voltage, the number of fuel cells in a fuel cell stack such as this may, for example, be 50 or more.
In order to make use of the potential difference which is generated during operation of the fuel cells that are joined together so as to form such a fuel cell stack, the circuitry of the fuel cell stack is provided with a load. In this case, a so-called pole plate, to which the electrical input and output cables can be connected, is arranged on each of the two outermost series-connected fuel cells in the fuel cell stack, in order to provide the electrical connection for the load.
Owing to the particular operating characteristics of such fuel cells and, in particular, with respect to the generation of just water as the only significant waste product, fuel cells are also particularly suitable for use for power supplies in intrinsically closed mobile systems, such as underwater vehicles. In this case, it is particularly advantageous that a comparatively high output current at a normal voltage level can be achieved with only restricted physical dimensions, in the form of comparatively high power density in a fuel cell arrangement.
Furthermore, particularly when used in underwater vehicles, the fuel, that is to say the substance which includes the hydrogen, can be produced in a comparatively compact form. In this case, pure oxygen may be used as the auxiliary substance or oxidant. In this case, the hydrogen may in particular be stored in hydride tanks.
When fuel cells are actually used in an underwater vehicle, it may be desirable to keep the signature that is emitted to the exterior, that is to say the externally detectable indications of operation of the underwater vehicle, particularly low. This signature may also include magnetic fields, which are produced by the currents that flow in and out during operation of fuel cells.